Inner cover for box annealing furnace



sept. 11, 1962 K. C. SHIPLEY 3,053,523

INNER COVER FOR BOX ANNEALING FURNACE Filed June 8, 1959 5 Sheets-Sheet1 Sgpt. 11, 1962 K. c. sHlPLEY 3,053,523

INNER COVER FOR Box ANNEALING FURNACE Filed June 8, 1959 5 Sheets-Sheet3 lh .Y ////l 1\ United States Patent 3,653,523 ENNER COVER FR EUXANNEALING FURNACE Kenneth C. Shipley, Chicago, lll., assignor to InlandSteel Company, Chicago, lll., a corporation of Delaware Filed .lune 8,1959, Ser. No. 818,724 t Claims. (Cl. 2665) This invention relates tothe annealing of steel. It is especially directed to apparatus forfacilitating the box annealing of stacked, coiled, flat-rolled steelproducts.

In the production of cold reducer, flat-rolled steel products inrelatively thin gauge, steel ingots are initially hot rolled into slabs4"7" thick. These slabs are further reduced in thickness by additionalhot rolling to produce a hat rolled product having a thickness of about1/16-1/s Depending upon consumer requirements, the hot rolled strip isfurther reduced in thickness by cold reduction to produce a steel sheethaving a smooth dense surface. Cold reduced, flat rolled products haveapplication as the base metal in the production of tin plate,`galvanized sheetings, electrical steels, or as `a material ofconstruction in the fabricating of a variety of sheet metal products.

In the cold working of the flatsrolled product structural changes occur`and it is necessary to process anneal the cold worked material atelevated temperatures to cause recrystallization of the cold workedstructure and to soften the steel. Although continuous annealingprocesses are available, a large proportion of cold rolled strip productis heat treated in a so-called box annealing process wherein coils ofthe cold rolled sheet steel are stacked in a heating furnace of asuitable design which has facilities for providing a controlledatmosphere for protecting the steel coil from oxidation during heatingand cooling, and covered by a light inner cover.

The annealing cycle, Which is carried out in the presence of acirculating controlled atmosphere gas such as nitrogen, or variousprepared gases containing nitrogen, and marketed under trademarks suchas DX, NX, I-INX, etc., or other compositions, consists of a heatingphase, a soaking phase, and a cooling phase. During the annealing phasethe cold rolled steel coils are heated to a temperature within the rangeof ll00l300 F. and held at this temperature. The heating phase is about24-48 hours and the soaking phase usually takes a total of about 4-24hours. Thereafter the charge of cold rolled steel coils is cooled withinthe heating furnace with a continued circulation o-f the atmospheregases. The cooling phase takes about 60-80 hours. Considerable time isrequired to insure the production of sheet steel having the desiredsoftness and ductility with -most of the time being consumed in theheating and cooling cycles.

In accordance with this invention there is provided an inner cover foruse in box annealing which can be utilized to facilitate the boxannealing process and decrease the heating `and cooling time of theannealing cycle 4without deleteriously affecting the quality of theannealed dat rolled product.

Referring to the drawings, FIGURE l shows a cross sectional View of aradiant tube-tired bell type furnace employed in heat treating coils offlat-rolled products in which is illustrated the use of the inner coverof this invention for enclosing the coils and in which a controlledatmosphere gas is circulated.

FIGURE 2 is a side elevation View of an illustrative inner coveremploying the features of this invention.

FIGURE 3 is a top view of the inner cover shown in FIGURE 2 with aportion of the end wall cut away.

FIGURE 4 is a fragmentary view of ran upper corner of the inner covershown in FIGURE l illustrating an illustrative means for reinforcing theouter casing Wall of the inner cover and facilitating its handling.

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FIGURE 5 is a fragmentary view of a bottom corner of the inner covershown in FIGURE 2 illustrating the flared open end and adjacentreinforcing means.

FIGURE 6 is a plan view of an illustrative diffuser and separator plateutilized for separating the stacked coils.

FIGURE 7 is a cross sectional view of the diffuser plate shown in FIGURE6 taken along line 7 7.

FIGURE 8 is a fragmentary view of a portion of the diffuser plate.

In the course of 'a box annealing operation the effect of the heattreating is influenced not only by the time of heating and thetemperature level maintained, but also by the atmosphere surrounding thematerial. In the annealing of coils of at rolled metal products in belltype furnaces inner covers are employed to enclose the coiled productand permit the circulation of a controlled Iatmosphere gas within theconfined area. In using inner covers, bell type furnaces having aremovable shell are employed to cover 'the hearth of the furnace. Thefurnace is charged by stacking coils of flat-rolled steel products onthe furnace hearth. An inner cover is placed over the stack of coils andsealed with a nely divided refractory material such as sand at thebottom. Then the furnace shell or portable heating unit is lowered overthe assembly. Provisions are made for supplying a constant stream of agas for control of atmosphere into the interior of the inner cover forcirculation around the coils. To aid in the distribution of the gaswithin the inner cover, separators are placed between the coils. Controlatmosphere gas is rst introduced into -the inner cover to purge theinterior. This gas also circulates during the heating, soaking, andcooling phases of the cycle.

In FIGURE l there is illustrated a typical bell type furnace 1o. Thisfurnace consists of a permanent base or stand 11 which is provided witha peripheral channel 12 and faced with a suitable refractory 13.Refractory 13 is provided with an annular groove 14 in which inner cover15 is placed. Heating cover 16, suitably lined with refractory materials17, is indirectly tired by means of radiant tubes 18 resting on suitablebrackets 19 mounted on the wall of heating cover 16. A suitable gas-aircombustion mixture is Supplied to conventional burner means, not shown,for supplying the heat required for the annealing cycle. Base fan 20,located in the base of the furnace, is employed for circulating the gasfor controlling the atmosphere within the inner cover in cooperationwith the baffled, diffuser element 21 and diffuser plate l22 resting onthe face of the base refractory 13. The gas is introduced into thesystem by suitable inlet and outlet means, not shown. A plurality offlat rolled product coils 25 are stacked on diffuser plate 22 withseparators 26 being placed between `each coil tO aid in the distributionof the controlled atmosphere gas inside the inner cover. Inner cover 15is placed over the stack of coils fwith tubular, hollow inner core 27positioned within the central openings of the coils with the open end ofinner cover 1S resting within annular grove 14. Inner cover 1S is sealedto the base 11 by means of a powdered refractory 28 piled about thelower end of inner cover 15. It will also be noted that the open end ofheating cover 16 rests within peripheral channel 12 which is packed witha suitable pulverized refractory 29 to effect the sealing of the heatingcover 16 with stand 11. Heating cover 16 is suitably braced and alsoprovided with lifting lug 30 which permits the heating cover to beremoved by a suitable hoisting device and set aside while the furnace isbeing charged with the coiled products. As hereinbefore mentioned, thebox annealing cycle requires a considerable amount of time. Because itis necessary for the heated coils to soak at the selected annealingtemperature for a time sufficient to effect the desired modification orimprovement in the softness or ductility of the product, the annealingcycle cannot be modified by reducing the time required for this phase.By employing the instant invention, however, it is possible to cut downon the time of the annealing cycle by as much as 25% or more, dependingupon the size of the coils and furnace, by reducing the time requiredfor the heating and cooling phases of the annealing cycle. This isprovided by employing an inner cover such as illustrated in FIGURE 2which employs an open ended axial tubular hollow inner core dependingwithin the central areas of the stacked coils. The inner core, whenpositioned, improves the circulation of the gas within the inner coverand decreases the time required for the stacked coils to be heatedthroughout to the desired soaking temperature as well as decreases thetime required for the coils to cool to a temperature which would permithandling after the soaking phase of the appealing cycle. In FIGURE 2 isillustrated a specific embodiment of an inner cover for use in theinstant invention. Inner cover 15 comprises an open ended outer tubularcasing 35. One end of tubular casing 35 is enclosed by means of end wall36 which is suitably mounted thereon by means of welding or otherconventional fastening expedients. To the other end of tubular casing 35is tted ared skirt 37 which in use rests within annular groove 14provided in base 11 of furnace 10. To facilitate Ithe handling of innercover 15 suitable reinforcing is provided. In the illustrative innercover a pair of top and bottom rings 38 and 39 fabricated fromrectangular cross sectional bar stock circumscribe casing 35 adjacenteither end thereof and are welded in place. End wall 36 is braced bymeans of grid support 40 which consists of a plurality of bars 41radially mounted on the inside of end Wall 36. Axially depending fromend wall 36 is open ended hollow tubular core 42. One end of innertubular core 42 is enclosed by means of that portion of end wall 36cooperating therewith. The free end of tubular core 42, however, isopen. The tubular core 42, which is preferably cylindrical, dependswithin casing 35 preferably `for a distance such that a restricted gaspassage is formed for the extent of the aligned central openings of thestacked coils. To improve the eifect of the inner tubular core 42 gasports 43 and 44 are provided at the end of tubular member 42 affixed toend Wall 36. These ports permit the ow of gases Within tubular core 42and preclude the formation of a dead air space at the closed end oftubular member 42 which might have a deleterious effect on the heatingefliciency of the inner cover of this invention.

In use the inner cover of this invention is placed over the stackedcoils to provide an outer and an inner cooperating annular gas channelformed, respectively, (l) between the inner wall of the outer casing andthe outer surface of the stacked coils and (2) the outer wall of tubularcore 42 and the inner surface of the central openings of the stackedcoils. The ratio of the cross sectional areas of the respective annulargas channels is selected such that the free flow area in the centeropenings of the coil stack is greatly reduced by means of the innertubular core. Accordingly, when a controlled atmosphere gas iscirculated within the inner cover a low pressure high velocity area iscreated in the annular gas channel formed in the central opening. Thispressure differential will draw the circulating gas for control ofatmosphere through the spacer plates provided between the stacked coilsto effect a more rapid heating and cooling of the coiled material,depending upon the temperature within the furnace.

To facilitate the placement of the inner cover it is preferred that theinner tubular member have a diameter with relation to the diameter ofthe central opening of the coil stack such that there is as much lateralclearance between the outer surface of the coil and the outer casing asthere is between the inner core surface of the coil and the wall of theinner tubular member. Using this design the inner cover can be readilypositioned without any interference between the inner tubular core andthe stack coils by using the outer casing as a guide in placing theinner cover in position.

As an illustration of the preferred ratios of the crosssectional areasof the outer and inner gas channels, for a coil of dat-rolled metalproduct having an outer diameter of 73 with a central opening of 24 thedimensions of the outer cover and inner tubular member are selected suchthat the ratio of the annular gas chambers should be within the range ofabout 2.4:1 to 125:1. A suitable inner cover for coils of this size isprovided if a lateral spacing of 4" is provided between the casing andcore elements of the cover and the coil surfaces adjacent thereto. It isapparent, however, that depending upon the particular designs employedas well as coil sizes, core sizes `and other factors, this ratio andillustrative lateral spacing can vary and other suitable ratios, as Wellas spacings, can be used in order to effect the objectives of thisinvention.

In the illustrative embodiment shown in FIGURE 2, cover plate wasdesigned for use -in the box annealing of coiled hot rolled products inwhich the coils had a maximum diameter of 73 with the central openinghaving a diameter of 24". The outer casing was fabricated from a lowcarbon, rimmed steel having the following analysis. Carbon 0.07% max.Manganese 0.32-0.42%. Phosphomis 0.015% max. Sulphur 0.30% max.

One half inch thick steel plate was employed in fabricating the outercasing as Well as the end wall. The outer casing was 14'71/2 long andwas circular in cross section having `a 69 O.D. The inner tubular hollowcore, which had an outer diameter of 12, was fabricated from 1/2 thickenameling iron and depended from the end wall to about 17 from theflared open end of the outer casing. A pair of diametrically opposed gasinlets were provided on the inner tubular member at the end adjacent theend wall. These gas inlet ports were semicircular holes having a 11/2radius. Suitable reinforcing in the form of 11/2 x 4" steel rings wasused adjacent the closed and open ends of the outer casing. Inassembling the various components by welding, care was taken to insurethat gas-tight seams were provided.

In use the inner cover is positioned over the stacked coils and heatsupplied to the furnace by a suitable heating system. During heating,heat is radiated from the furnace heating tubes toward the inner cover.Concurrently heat accumulates at the top of the furnace and lnner cover.A portion of this heat would be conducted down the core of the innercover of this invention and as the outer casing and inner tubular corebecome hot, heat is radiated simultaneously toward both the outer andlnner layers of the coiled fiat rolled products.

With the tubular core disposed within the central open- 1ng, the propertiow of convection currents, which are the principal means fortransferring heat from the inner cover to the coils, is effected. Theprepared control atmosphere circulates inside the inner cover in a pathwhich goes up the outside of the coiled stack yand down the centralopening of the stack with a portion of the hot atmosphere beingdistributed through separator plates placed between each coil. Byemploying the inner cover design of this invention the pressure andvelocity differential of the circulating gas system is increased. Thepressure dlfferential is important because it causes the circulatingatmosphere to pass through the separator plates heating or cooling theinner layers of the coils, depending upon the phase of the annealingcycle. Increasing the velocity of the hot gas passing any portion of thecoiled surfaces also aids in transferring heat to the coil because withthe increased velocity any insulating surface lm is removed moreeciently. In cooling the coils the system is very much the same with theflow of heat being in the opposite direction.

In FIGURES 6-8 is shown a typical separator plate 50 which consists ofan annular ring 51 having a plurality of involute vanes 52 equallyspaced on the outer diameter and inner diameter of annular ring 51. Tofacilitate the handling of the separator plate a lifting ring 53 ofsuitable dimensions is positioned within the opening of annular ring 51and welded thereto. The opening in the lifting ring has about the samearea as the transverse cross sectional area of the core opening of acoiled product with which it is used.

In fabricating the inner cover of this invention it is preferred thatheat resisting alloys be utilized and that plate thicknesses be suicientto minimize damage to the inner cover upon handling. Accordingly, theouter casing and inner core should be fabricated from materialspreferably having a thickness of lle/2%. 'In order to avoid heatdistortion of the inner tubular member an enameling grade steel can beused because these steels can be employed at the annealing temperatureswithout distortion. Although it is preferred that the outer casing andinner core have substantially circular cross sections, it is apparentthat other tubular configurations can be employed. In addition it is notnecessary that the component elements of the heating cover be fabricatedfrom ilat plate, and other types of surfaces, such as corrugated, can

be used.

It is apparent that the instant invention has application in a varietyof annealing processes wherein the steel or other material to beannealed is in coiled yform and arranged in stacks within a suitablebell-type box annealing furnace. In employing the instant invention toimprove the circulation of controlled atmosphere gases within the innercover by modifying the velocity and pressure differentials within theinner cover, variations in the specifically described inner cover can bemade without `departing from the scope of the invention. It is thereforeintended that the subject invention, which has other equivalents andmodifications, be defined by the claims appended hereto.

I claim:

l. An inner cover for box annealing a stack of atrolled metal productcoils having a central opening, the coils in said stack being separatedby separator plates placed between said coils, which comprises anopen-ended, outer tubular casing, having a solid outer wall, a solid endwall enclosing one end of said casing, an open-ended inner tubularhollow core depending from said end wall and substantially coaxial withsaid outer casing, said inner core being provided with inlet portsadjacent the terminal end thereof attached to said end wall and with thesidewall portion of said core below the terminal end being free fromopenings, said cover adpated to cover said coils with said corepenetrating said central opening with said outer casing cooperating withthe outer surface of said coil to form a first gas passage for theupward iiow of a controlled atmosphere gas, and said inner corecooperating with the inner surface of the central opening of said coilto form a second gas passage for the downward flow of a controlledatmosphere gas, first and second passages being interconnected by meansof said spacer plates and a header space at the top of said inner cover,the ratio of the cross-sectional area of said first passage to thecross-sectional area of the second passage being sufficiently high toprovide a low pressure, high velocity area in said second passage withrespect to said first passage.

2. An inner cover for box annealing a stack of flatrolled metal productcoils having a central opening, the coils in said stack being separatedby separator plates placed between said coils, which comprises anopenended, outer tubular casing, having a solid outer wall, a

solid end wall enclosing one end of said casing, an openended innertubular hollow core having a substantially circular cross sectiondepending from said end wall and substantially coaxial with said outercasing, said inner core being provided with inlet ports adjacent theterminal end thereof attached to said end wall and with the side- Wallportion of said core below the terminal end being free from openings,said cover adapted to cover said coils with said core penetrating saidcentral opening with said outer casing cooperating with the outersurface of said coil to form a first gas passage for the upward iiow ofa controlled atmosphere gas, and said inner core cooperating with theinner surface of the central opening of said coil to form a second gaspassage for the downward flow of a controlled atmosphere gas, first andsecond passages being interconnected by means of said spacer plates anda header space at the top of said inner cover, the ratio of thecross-sectional area of said -first passage to the cross-sectional areaof the second passage being sufhciently high to provide a low pressure,high Velocity area in said second passage with respect to said firstpassage.

3. An inner cover for box annealing a stack of atrolled metal productcoils having a central opening, the coils in said stack being separatedby separator plates placed between said coils, which comprises anopen-ended, outer tubular casing, having a solid outer wall, a solid endwall enclosing one end of said casing, an open-ended inner tubularhollow core depending from said end wall and substantially coaxial withsaid outer casing, said inner core being provided with inlet portsadjacent the terminal end thereof attached to said end wall and with thesidewall portion of said core below the terminal end being free fromopenings, said cover adapted to cover said coils with said corepenetrating said central opening with said outer casing cooperating withthe outer surface of said coil to form a first gas passage for theupward flow of a controlled atmosphere gas, and said inner corecooperating with the inner surface of the central opening of said coilto form a second gas passage for the downward flow of a controlledatmosphere gas, first and second passages being interconnected by meansof said spacer plates and a header space at the top of said inner cover,the ratio of the cross-sectional area of said first passage to thevcross-sectional area of the second passage being sufficiently high toprovide a 10W pressure, high velocity area in said second passage withrespect to said first passage, said second passage having at least thesame lateral width as said first passage.

4. An inner cover in accordance with clairn 3 in which said inner corehas a substantially circular cross section.

5. An inner cover for box annealing a stack of flatrolled metal productcoils having a central opening, the coils in said stack being separatedby separator plates placed between said coils, which comprises anopen-ended, outer tubular casing, having a solid outer wall, a solid endwall enclosing one end of said casing, an open-ended inner tubularhollow core having a circular cross section depending from said end walland substantially coaxial with said outer casing, said casing, end Wall,and core being fabricated from heat resistant materials of constructionwith said core being fabricated from enameling grade steel, said innercore being provided with inlet ports adjacent the terminal end thereofattached to said end wall and with the sidewall portion of said corebelow the terminal end being free from openings, said cover adapted tocover said coils with said core penetrating said central opening withsaid outer casing cooperating with the outer surface of said coil toform a first gas passsage for the upward ilow of a controlled atmospheregas, and said inner core cooperating with the inner surface of thecentral opening of said coil to form a second gas passage for thedownward flow of a controlled atmopshere gas, first and second passagesbeing interconnected by means of said spacer plates and a header spaceat the top of said inner cover, the ratio of the cross-sectional area ofsaid first 7 passage to the cross-sectional area of the second passagebeing sufficiently high to provide a low pressure, high velocity area insaid second passage with respect to said irst passage, said secondpassage having at least the same lateral width as said first passage.

6. In combination, a bell type box annealing furnace having a hearth andremovable heating cover, a stack of a plurality of atrolled metalproduct coils positioned on said hearth and having a central openingcoextensive with said stack, separator plates disposed between each coillayer, and an inner cover resting on said hearth and enclosing saidstack which comprises an open-ended, outer tubular casing, having asolid outer wall, a solid end wall enclosing one end of said casing, anopen-ended inner tubular hollow core depending from said end wall andsubstantially coaxial with said outer casing, said inner corepenetrating said central opening and substantially traversing said stackand being provided with inlet ports adjacent the terminal end thereofattached to said end wall and with the sidewall portion of said corebelow the terminal end being free from openings, said outer casingcooperating with the outer surface of said coil to form a first gaspassage for the upward flow of a controlled atmosphere gas, and saidinner core cooperating with the inner surface of the central opening ofsaid coil to form a second gas passage for the downward flow of a con*trolled atmosphere gas, first and second passages being interconnectedby means of said spacer plates and a header space at the top of saidinner cover, the ratio of the cross-sectional area of said rst passageto the crosssectional area of the second passage being sufficiently highto provide a low pressure, high velocity area in said second passagewith respect to said first passage.

7. ln combination, a bell type box annealing furnace having a hearth andremovable heating cover, a stack of a plurality of flat-rolled metalproduct coils positioned on said hearth and having a central openingcoextensive with said stack, separator plates disposed between each coillayer, and an inner cover resting on said hearth and enclosing saidstack which comprises an open-ended, outer tubular casing, having asolid outer wall, a solid end wall enclosing one end of said casing, anopen-ended inner tubular hollow core having a substantially circularcross section depending from said end wall and substantially coaxialwith said outer casing, said inner core penetrating said central openingand substantially traversing said stack and being provided with inletports adjacent the terminal end thereof attached to said end wall andwith the sidewall portion of said core below the terminal end being freefrom openings, said outer casing cooperating with the outer surface ofsaid coil to form a rst gas passage for the upward flow of a controlledatmosphere gas, and said inner core cooperating with the inner surfaceof the central opening of said coil to form a second gas passage for thedownward tlow of a controlled atmosphere gas, first and second passagesbeing interconnected by means of said spacer plates and a header spaceat the top of said inner cover, the ratio of the cross-sectional area ofsaid first passage to the cross-sectional area of the second passagebeing suiciently high to provide a low pressure, high velocity area insaid second passage with respect to said iirst passage.

8. In combination, a bell type box annealing furnace having a hearth andremovable heating cover, a stack of a plurality of at-rolled metalproduct coils positioned on said hearth and having a central openingcoextensive with said stack, separator plates disposed between each coillayer, and an inner cover resting on said hearth and enclosing saidstack which comprises an open-ended, outer tubular casing, having asolid outer wall, a solid end wall enclosing one end of said casing, anopen-ended inner tubular hollow core having a substantially circularcross section depending from said end wall and substantially coaxialwith said outer casing, said inner core penetrating said central openingand substantially traversing said stack and being provided with inletports adjacent the terminal end thereof attached to said end wall andwith the sidewall portion of said core below the terminal end being reefrom openings, said outer casing cooperating with the outer surface ofsaid coil to form a iirst gas passage for the upward ow of a controlledatmosphere gas, and said inner core cooperating with the inner surfaceof the central opening of said coil to form a second gas passage for thedownward flow of a controlled atmosphere gas, first and second passagesbeing interconnected by means of said spaced plates and a header spaceat the top of said inner cover, the ratio of the cross-sectional area ofsaid first passage to the cross-sectional area of the second passagebeing sufliciently high to provide a low pressure, high velocity area insaid second passage with respect to said first passage, said secondpassage having at least the same lateral width as said first passage.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Steel, July 21, 1947, pages 130, 132, 134, 137 and 140 reliedon.

